Field of the Invention
This invention relates to a fixing device. More specifically, this invention relates to a fixing device equipped with a first and a second rotating bodies to fix a toner image onto a sheet, by holding and conveying the sheet at a nip portion.
Description of the Related Art
As electrophotography image forming apparatuses, there are a MFP (Multi Function Peripheral) with a scanner function, a facsimile function, a copying function, a function of a printer, a data transmitting function and a server function, a facsimile device, a copying machine, a printer, and so on.
An image forming method of a generic image forming apparatus is as follow. An image forming apparatus electrostatically charges a photo conductor by using an electrostatic charging device. An electrostatic latent image is formed on the photo conductor by laser beams emitted from an expose device. The image forming apparatus forms a toner image by developing the electrostatic latent image by using a developing device. The toner image is transferred onto a sheet by using a transfer roller. The image forming apparatus forms the image onto a sheet by fixing the toner image onto the sheet by using a fixing device.
As for a fixing device of an image forming apparatus, to improve separability of sheets (especially, thin paper) and a gloss memory, a method to make a difference between velocities of surfaces of two parts which pinch and convey a sheet for fixing (for example, a lower pressure roller and a fixing belt) is proposed. According to this method, a sheet is separated from a fixing belt by shearing force which occurs between a toner image on the sheet and a surface layer of the fixing belt. Such the technique is disclosed in the below Document 1, for example.
Document 1 below discloses a structure in which an upper pressure roller and a lower pressure roller contact with each other via a fixing belt with pressure. A velocity difference is configured between a velocity of the surface of the lower pressure roller and a velocity of the surface of the fixing belt at a fixing nip portion where a sheet is pinched and conveyed. Under the velocity difference, the lower pressure roller and the fixing belt rotate, so that the paper sheet is fed.
[Document(s)]
[Document 1] Japan Patent Publication No. 2014-81610
Generally, the upper pressure roller is rotationally driven only for the purpose of heat leveling of the fixing belt when separating. However, according to the technique of Document 1, the upper pressure roller should be rotationally driven, keeping the configured velocity difference between the velocity of the surface of the upper pressure roller and the velocity of the surface of the lower pressure roller, when feeding paper. In consequence, the driving control system for the upper pressure roller gets complex, as compared with the conventional system, so that the manufacturing cost increases. On the other hand, according to the technique of Document 1, even though the driving control system gets complex and the manufacturing cost increases, the separability of sheets is unsatisfactory.